Initial Stages Of Rifting Research Articles

Temporal evolution of mantle sources during continental rifting: The volcanism of Djibouti (Afar)

Magmatism occurred almost continuously over the past 25 m.y. in the Republic of Djibouti. Lavas are mainly basic to intermediate with some rhyolites. Large chemical and isotopic variations among the volcanic series are interpreted in terms of mantle source heterogeneity. Crustal contribution is only evidenced in the oldest rhyolites emplaced during the initial stages of rifting. Excluding these old rhyolites, a clear evolution through time of the mantle sources is observed in relation to rifting. Three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (87Sr/86Sr ≈ 0.706, 206Pb/204Pb ≈ 17.9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (high U/Pb ratio)‐type reservoir, and (3) a depleted mantle. As rifting goes on, there is an increasing contribution of an HIMU‐type mantle source. It is attributed to the influence of a mantle diapir (Afar plume) thermally eroding the subcontinental lithosphere. The geochemical characteristics of 9 to 1 Ma old lavas, erupted after the strong increase of spreading rate in Afar, reflect this evolution of mantle sources. The influence of the mantle plume is most prominent in the northern youngest lavas (<1 Ma), particularly Manda, characterized by the strongest HIMU signature (87Sr/86Sr ≈ 0.7035, 206Pb/204Pb ≈ l9.2). The contribution of the depleted mantle component originating from the asthenosphere is best recognized in the young (<4 Ma) lavas, particularly Tadjoura and Asal lavas (3 to 1 Ma). The evolution of Djibouti lava sources through time may be accounted for by the recent models developed for plume structure.

Volcanism and sedimentation in a rifting island-arc terrain: an example from Tonga, SW Pacific

Abstract Scientific drilling of narrow sub-basins within the Lau back-arc basin system of the SW Pacific has recovered uppermost Miocene to Recent volcaniclastic sediment and pelagic nannofossil oozes. Pliocene sediment gravity flow and turbidite sands from the western Lau Basin indicate a local source for the sediment, probably intrabasinal seamount volcanoes active during the initial stages of arc rifting. Derivation of abundant material from either the remnant volcanic arc (Lau Ridge) or the new Tofua arc is ruled out by the rugged topography of the basin and the proximal nature of the facies. Sediments from the Tonga platform, adjacent to the present day Tofua arc, indicate a peak in volcanic activity prior to and during the generation of the first back-arc basin crust at 5.25 Ma. A 2.0 Ma hiatus in arc volcanism on the trench side of the basin after rifting was brought to an end by the foundation of the Tofua arc at 3.0 Ma (Late Pliocene). On the basis of the sedimentary, geochemical and seismic data it is suggested that basin rifting involved an initial stage of extension of the original island arc, accompanied by volcanism in the form of major seamount volcanoes within the basin. These produced volcanic ash by submarine eruption, which was then reworked into adjacent sub-basins by slumping, gravity flow or turbidity current. Basin opening proceeded with a trenchward migration of extension and volcanism with time. This system was disrupted by the southward propagation of the Eastern Lau Spreading Centre into the southern Lau Basin at 1.5–1.0 Ma. This resulted in extension and volcanism being concentrated along the median valley of the spreading centre and a cessation in explosive volcanism of wide compositional range. Sedimentation in the Lau Basin since that time has been principally pelagic with minor ash layers mostly derived from the Tofua arc.

Tectonic Patterns and Cenozoic Basalts in the Western Margin of the South China Sea: ABSTRACT

The allochthonous fragments - Indosinia, Sibumasu, East Malaya, and Southwest Kalimantan - rifted from Gondwanaland and drifted northward. Indosinia collided with the Yangzi-Huanan terranes in Devonian or Early Carboniferous period and became part of the East Asia continent. Sibumasu collided· with East Asia continent and East Malaya during the Indosinian Orogeny (220-200 Ma). The Southwest Kalimantan terrane probably rifted from the northeast margin of Indosinia in the Cretaceous. ·At about 50 Ma the collision of the Indian continent into the Eurasian continent led to the fragmentation of Asia and was followed by the opening of the Andaman Sea, the clockwise rotation of the Indochina plate, and the rifting and the opening of the South China Sea. The Late Cretaceous alkaline intrusions in the Red River area of northern Vietnam formed during initial stage of rifting of the South China Sea. The Indian-Eurasian collision has successively pushed the Indochina Peninsula in the east­ southeast direction. Most of the Middle Tertiary movements probably occurred along the left-lateral Red River, Tonle Sap-Mekong faults concurrent with the opening of most of the eastern South China Sea. The extensional tectonics along these predominantly strike-slip faults may be responsible for the Pliocene­ Pleistocene alkaline basalts, which extend from the Mekong Delta northwestward into Thailand. Some basalt outcrops in Thailand, Kampuchea and Vietnam appear to trend north-south. The composition of basalts ranges from tholeiitic to alkalic. The basanitoid basalts contain abundant megacrysts of zircon, ruby, sapphire, and spinel. The megacrysts probably originated through assimilation of metamorphic rocks deep in the continental crust. The cores from the South China Sea indicate that the increase in subsidence rate appears in wells during the Quaternary. It may be related to the increase in Quaternary basalt and high heat flow in the western part of the South China Sea basin.

The initiation of rifting at constant tectonic force: Role of diffusion creep

We have investigated the effect of diffusion creep on lithospheric extension using a one‐dimensional numerical model that assumes a constant force is available to drive extension. The model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength. However, such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep. We consider the evolution of strain rate and temperature at the center of an idealized pure shear rift. The strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses. Given an initial thermal condition and an assumed lithospheric rheology, the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate. The force at which this condition is met we will term the critical force. Diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force. We speculate that during the initial stages of continental rifting, grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength.

Evolution of a conjugate passive margin pair in Mesozoic southern Turkey

In the western Tauride belt of southern Turkey, a north pointing cusp (Isparta Angle) occurs along the Mesozoic carbonate axis and encompasses passive continental margin sequences, rift assemblages, mantle peridotites, an ophiolite complex, and platform fragments. The contacts between these tectonostratigraphic units are commonly tectonic with the carbonate platforms forming the autochthonous basement to the west and east directed thrust sheets both in the west and the east. Rift assemblages consist of chert, pelagic limestone, mudstone, sandstone, and alkaline lavas ranging in age from Upper Triassic to Upper Cretaceous and locally include serpentinite lenses. Mantle peridotites are commonly harzburgitic, cut by numerous doleritic dikes, and underlain by a syntectonic mélange. The ophiolite complex occurs in the south, adjacent to and overlying the western platform (Bey Daglari), and consits mainly of peridotites and cumulate gabbros with a poorly developed dike complex and rare extrusive rocks. Platform fragments are represented by north‐south trending slivers of limestones that have affinity with the western platform. They are technically overlain by the rift assemblages and/or mantle rocks within the Isparta Angle. Rock types and associated structures within the Isparta Angle indicate a progressive evolution from continental rifting and passive margin development to ocean basin formation in the Tauride belt of the Neotethyan realm during Triassic to Late Cretaceous time. The pronounced asymmetry in the crustal architecture of the conjugate margins suggests that initial stages of rifting and subsequent extension were probably facilitated by crustal failure along an east dipping (in present coordinates) low‐angle fault. Continued rifting and associated crustal thinning resulted in upwelling and emplacement of the mantle beneath the lower plate and in partial melting of this elevated mantle generating alkaline lavas. Further extension in the south established a spreading system along which Cretaceous oceanic crust, now represented by the Tekirova ophiolite, was created. Contractional deformation in Late Cretaceous to Late Tertiary time caused telescoping of rock units within the Isparta Angle and emplacement of thrust sheets onto the platform carbonates both in the east and the west. The opening and closure stages of the ocean basin within the Isparta Angle were diachronous while the bounding carbonate platforms were probably revolved around a pivoting pole located at or near the apex of this sharp bend in the Tauride belt.

Magmatism and rifting in Western and Central Africa, from Late Jurassic to Recent times

Mesozoic-Cenozoic magmatic activity in West and Central Africa is reviewed, with particular emphasis on the relationship between Mesozoic magmatism, major phases of continental rifting and the opening of the Equatorial Atlantic. It is suggested that during the initial stages of rifting, the activity of a mantle plume, the St. Helena hotspot, may have been important in weakening the lithosphere across the region. Evidence for magmatism concurrent with the onset of rifting in some basins supports such an active rifting model. Magma compositions range from alkali to tholeiitic basalts and their differentiates. Transitional to tholeiitic basalts are comparatively rare and are generated by greater degrees of partial melting, at probably shallower mantle depths, than associated alkali basalts. In some instances their occurrence may be correlated with higher amounts of lithospheric extension. However, in other instances they appear early in the rift sequence when overall amounts of extension were small. These tholeiitic basalts often have geochemical characteristics dominated by an ancient sub-continental lithosphere isotopic signature, which may have been introduced by crustal contamination. Cenozoic magmatism of alkaline affinity is widespread in West and Central Africa. In many instances, sites of activity appear to be structurally controlled by pre-existing basement fractures/lineaments of Mesozoic-Precambrian age. Most of the volcanic fields lie outside the boundaries of the Cretaceous rifts and many are associated with broad basement uplifts. However, there has also been a rejuvenation of tectono-magmatic activity within several of the Cretaceous rift basins during the Neogene. The parental magmas are considered to be generated mainly by partial melting of a zone at the base of the sub-continental lithosphere, which was variably metasomatised by the activity of mantle plumes beneath the African plate during the Mesozoic.

Crustal structure and magmatism of North Atlantic continental margins

Enormous variations exist along rifted margins of the North Atlantic in the magmatic activity that accompanied continental breakup. The North Atlantic margins exhibit a full range of magmatic behaviour. One end of the spectrum (commonly termed ‘volcanic’ margins) is found in the northern North Atlantic off Greenland, Norway and northwest Britain, where huge volumes of igneous rock were added to the continental crust as it rifted, and the adjacent oceanic crust is considerably thicker than normal. The other end of the spectrum (so-called ‘non-volcanic’ margins) occurs further south off France and Spain, where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust. Results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced. The volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle, and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere. Large quantities of igneous rock were produced as the continents broke up to form the North Atlantic where the newly initiated Iceland plume brought abnormally hot mantle beneath the rift. Much smaller quantities of melt were produced further south, where mantle temperatures were normal during rifting. The distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust. Where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years, as is common during the initial stages of rifting, then the rising asthenospheric mantle has time to cool by conduction. Heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region. As a consequence of the reduced mantle temperatures, considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts. This explains the abnormally thin oceanic crust found adjacent to the non-volcanic rifted continental margins in the North Atlantic. As the new ocean widens, the spreading centre becomes isolated from the continental margins on either side by newly formed, warm oceanic lithosphere, and the rate of rifting generally increases. The oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces, and all the melt formed by decompression of the mantle solidifies close to the spreading axis. Basaltic melts can intrude continental crust far more easily than oceanic crust. Basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts. This redistributes melt laterally towards the continent in both volcanic and, to a lesser extent, in non-volcanic margins and may be a contributory factor in leaving unusually thin crust in the first-formed ocean immediately adjacent to some rifted margins. It also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the Rockall area northwest of Britain.

Magmatism and continental rifting during the opening of the South Atlantic Ocean: a consequence of Lower Cretaceous super-plume activity?

Abstract Two large-scale mantle plumes, whose present-day foci are close to the oceanic islands of Tristan da Cunha and St Helena, appear to have played a significant role in the initial stages of rifting between Africa and South America during the Early Cretaceous opening of the South Atlantic Ocean. They may represent the initial burst of a super-plume event which generated extensive oceanic plateaux in the Pacific and Indian oceans. The recent volcanic products of Tristan da Cunha and St Helena have near endmember Sr-Nd-Pb isotopic characteristics (EM I and HIMU) in the spectrum of ocean basalt isotopic compositions. These isotopic signatures are recognised for more than 100 Ma in the plume-related magmatic products and therefore appear to be a long-lived feature of the plume source. The history of rifting and magmatism in West and Central Africa/NE Brazil and in southern Brazil, above the broad heads of the initial starting plumes between 145 Ma and 130 Ma, strongly suggests that there are different physical differences between the two plumes in addition to chemical ones. The St Helena plume appears to have been much weaker and cooler, with a smaller buoyancy flux. The hotter Tristan plume has generated voluminous flood basalts volcanism in the Paraná basin of Brazil and appears to be associated with continental break-up within a few million years of the plume head impinging on the base of the lithosphere. In contrast, in West and Central Africa, rifting above the St Helena plume, associated with small volumes of alkaline-transitional magmatism, spans an extended period of 30–40 Ma before break-up occurs in the Equatorial Atlantic.

Triassic “Gondwana” granites of the Gastre district, North Patagonian Massif

ABSTRACTGranites of the Batholith of Central Patagonia were emplaced into late Precambrian metamorphic basement rocks and Palaeozoic orthogneisses (here dated by Rb–Sr whole-rock errorchrons at 346 ±35 Ma and 267 ±27 Ma), and are in fault-contact with younger volcanic rocks, mostly andesites. Two main suites of granites are recognised: both are much younger than their previously-supposed Late Palaeozoic age. The Gastre Suite is composed predominantly of hornblende-biotite granodiorite and monzogranite, often slightly foliated, and has yielded a Rb-Sr whole-rock isochron age of 220 ±3 Ma. The Lipetrén Suite includes biotite monzogranite and leucogranite, grading into quartz-feldspar porphyries and felsites, and has been dated at 208 ±1 Ma. A minor granodioritic unit yields an apparently Middle Jurassic age of 172 ±15 Ma. Textural evidence and hornblende geobarometry confirm that these are high-level sub-volcanic intrusions. Metaluminous compositions are common in the Gastre Suite, but are subordinate to highly siliceous (>70% SiO2) and peraluminous varieties in the Lipetrén Suite. Despite this compositional bias, the granites are almost entirely calcalkaline and I-type, and have volcanic-arc rather than intraplate or collisional trace element characteristics. Initial87Sr/86Sr ratios of 0·706 and –2·5 are also relatively primitive and are thought to indicate a juvenile crustal contribution (Nd “depleted mantle” model ages are less than 1,000 Ma).The Triassic “Gondwana” magmatic episode is thus not an expression of Permo-Triassic collision of an allochthonous Patagonian terrane with the rest of southern S America, but may be related to the initial stages of supercontinent rifting.

Volcanism and tectonism in western Mexico: A contrast of style and substance

The Jalisco block of southwestern Mexico records the initial stages of continental rifting superimposed on a convergent continental margin. Pliocene- Quaternary eruptions in north-northwest-trending graben systems within the block produced extraordinarily diverse lava types, including minette, leucitite, absarokite, and andesite. Interspersed with these are several basaltic shield volcanoes that have compositional similarities to lavas from oceanic islands. Evidence of Miocene Basin and Range extension, which is seen around much of the Gulf of California, is found only along the northern margin of the Jalisco block. Large volumes of alkali basalt were also erupted during the late Miocene at several locations along this northern margin. We propose that Pliocene- Quaternary extension and associated volcanism within the confines of the Jalisco block are consequences of complex plate-boundary reorganizations that are causing the block to rift away from the Mexican mainland

The Colorado River Extensional Corridor: An Analog for the Early Stages of Passive Margin Formation

Structural asymmetry of many passive continental margins has recently led to the concept that the initial stages of rifting (extension and thinning of continental crust) occur on large-scale, low-angle normal (detachment) faults. Similarities in geometry and evolution of passive margins and detachment-related basins in the Colorado River extensional corridor (CRec), California-Arizona, suggest that development of the CRec resembles early stages of passive margin formation. Two major sets of high-angle synrift faults have been described from the upper plates of passive margins - rotational normal faults and an orthogonal set of transfer faults. Transfer faults, oriented approximately parallel to extension direction, divide an extending terrane into structural blocks which are tilted by the rotational normal faults. Similarly, the generally rectilinear shapes of syn-detachment sedimentary basins or subbasins within the CRec suggest control by two orthogonal fault sets. One set, oriented perpendicular to tectonic transport direction, broke the region into half-graben. These faults, and thus the basins, terminated against faults oriented approximately parallel to tectonic transport direction. The CRec basins experienced a two-stage history, with an early stage of active extension, which brought mid-crustal rocks to near-surface positions, followed by an adjustment stage, during which topographic relief between the range cores andmore » the basins increased. If this adjustment resulted from differential subsidence within the extended region rather than from uplift of range cores, then the two-part evolution resembles that of classic rifted margins: an initial active stage followed by subsidence.« less

Geological and geochemical studies of the Sierra del Morro-Oeste (San Luis Province, Argentina): Meta-sediments and meta-volcanics from a probable back-arc setting

The Sierra del Morro-Oeste covers an area of some 40 km 2 in the northeastern part of San Luis Province, Argentina. This area is an integral part of the scheelite- and wolframite-bearing crystalline basement of the Sierras Pampeanas. A “characteristic rock sequence”—several tens of meters thick—is concordantly embedded between uniform country rocks (quartz-oligoclase/andesine micaschists and gneises); it includes layered alternations of banded amphibolites, hornblende-epidote schists, scheelite-bearing calc-silicate rocks, lenses of barren marble, and, locally, tourmaline-bearing micaschists. Chemical analyses of amphibolite samples allow these metamorphic rocks to be interpreted as derivatives of a tholeiite-basaltic volcanism. The amphibolites are subdivided into two groups (A and B) because of significant differences in their chemical composition. These differences are related to hydrothermal seafloor alteration. This sea-water alteration has been most effective in the youngest tholeiite-basaltic lava extrusions. The derivatives of the submarine altered tholeiite-basaltic lavas (amphibolites of Group B) can therefore be interpreted as younger in comparison with the primary lavas of the non-altered amphibolites of Group A. The extrusion of the tholeiite-basaltic lavas obviously took place during an initial stage of back-arc basin rifting on continental crust.

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge, Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ɛNd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (ɛNd=+7.5−+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ɛNd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.

REE, Sm [sbnd]Nd and U [sbnd]Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): evidence for crustal contamination

A geochemical and geochronological study of the Alpine External Crystalline Massifs (AECM) of Aiguilles Rouges, Belledonne and Argentera was undertaken in order to constrain the geodynamic evolution of this segment of the Variscan foldbelt. Another aim of the study is to characterize the behaviour of isotopic markers, in particular the U Pb zircon system, under high-grade metamorphic conditions. The whole-rock geochemistry of eclogites and amphibolites was investigated using major and trace element (including the REE) analytical techniques; isotopic studies were performed by application of the Sm Nd whole-rock and U Pb zircon methods. In terms of regional geological history, the early development of metamorphic and magmatic activity in the AECM is typical of the extensional tectonic regime observed throughout the Variscan foldbelt during the Cambro-Ordovician (i.e. basic magmatism dated at 475-450 Ma). The composition of the metabasic rocks is closely similar to tholeiites emplaced into thinned continental crust which are generally associated with the initial stages of oceanic rifting. The source regions for these metabasics are characterized by initial ε Nd values between +6 and +8, suggesting depleted mantle sources influenced by a weak crustal component and/or the existence of a metasomatised lithosphere. The multi-stage eclogite-facies metamorphism is dated at 425-395 Ma (i.e. Silurian). An application of the U Pb method, associated with the artificial abrasion of zircon grains, has led to the recognition of a weak crustal contamination in the metabasic protoliths. This is implied by the Archaean and Lower Proterozoic upper intercepts on Concordia—devoid of geological significance—which reflect the presence of a pre-existing basement to the AECM. The U Pb zircon results have yielded upper and lower intercepts which represent, in different cases, either an ancient inherited component, the age of magmatism, eclogite-facies metamorphism or even the effects of late-stage recrystallization. The interpretation of these intercepts is rather ambiguous, but can be corroborated by zircon morphology studies and a comparison with the results obtained by other isotopic methods.

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift‐bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle‐derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA‐type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)‐depleted (PREMA) mantle hybridization. During the Holocene, proto‐oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust‐mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA‐type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension‐related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction‐driven lithospheric recycling.

The Cretaceous paleorift in northwestern Argentina: A petrologic approach

The development of the Cretaceous-early Eocene basin of northwestern Argentina can be divided into three main magmatic phases on the basis of preliminary petrologic data. The oldest phase (130-100 Ma) is divided into an early stage of anorogenic plutonism, with subalkaline, alkaline, and minor peralkaline granitic intrusives, and a volcanic stage in which alkaline rocks characterized by trachytes, basanites and foidites prevail. The second phase (80-75 Ma) is characterized by an alkaline suite with basanites, hawaiites and tephriphonolites. The last phase (65-60 Ma) consists of lamproitic sills and basic lava flows. The two first phases correspond, respectively, to prerifting and initial rifting stages. According to the magmatic, tectonic, and sedimentary features observed, it is suggested that this basin is a foreland paleorift, of low volcanicity type, that developed along the western side of South America from the Early Cretaceous to the Eocene—at which time the rift basin was closed by the Incaic diastrophic phase.

A hot-spot model for early Tertiary volcanism in the N Atlantic

Summary The initial stages of rifting of the N Atlantic were accompanied by massive extrusion of volcanic rocks and the accretion to the lower crust of even greater volumes of igneous rock. Emplacement of the bulk of the igneous rocks along the 2000 km long continental rift occurred in as little as 2–3 My. The pattern and timing of magmatic activity can be explained by partial melting of anomalously hot asthenosphere as it welled up to fill the space left by the stretched and thinned continental lithosphere. Asthenosphere temperatures were elevated regionally by 100–150°C over a 2000 km diameter region by the mushroom-shaped head of hot material fed by a mantle plume, which at present is centred beneath Iceland.

A model for rift development in Eastern Africa

A tectonic survey has been completed in Eastern Africa, using remote sensing as well as structural analysis in the field. New data have been collected in the eastern and western branches of the East African Rift system. The geomorphology suggests that the main deformation occurred in the two principal branches of the East African Rift system, but compilations of earthquake data show that recent movements are widespread over a large area in Eastern Africa. Many ancient structures have been reactivated during Neogene times, especially large Precambrian lineaments which are evident on satellite imagery. The Tanganyika–Rukwa–Malawi lineament, along its segment between Lakes Tanganyika and Malawi, now serves as an intracontinental transform zone, with folds and transcurrent faults. The Aswa lineament is, at the present time, only partly reactivated between Lake Mobutu and the Gregory Rift, and also along its southeastern continuation to the Indian Ocean. In many areas, tension gashes or striations on the slickensides have given the local palaeostress orientation. It appears to be changing with time, but the most typical orientation indicates horizontal extension striking NW–SE or WNW–ESE. At the earliest stage of rifting compression was horizontal, then it later became vertical. Compilation of earthquake foci from publications, confirms these results. All these observations together with data concerning the geometry of the deformation, as well as the mechanisms of the Cenozoic intracontinental deformation, implies a near NW–SE movement of the Somalian block, relative to the African continent. A dynamic tectonic model for the rift opening and propagation of the main fracturing across Africa is proposed. It suggests that opening is more important in the north than in the south. Four stages can be identified in the evolution of the East African Rift. The pre-rift stage corresponds to a dense fracturing, characterized by strike-slip faults, while a shallow but wide depression formed, and open tension gashes gave way to tholeiitic volcanism. The initial rifting stage is recognized by oblique-slip faults, bounding tilted blocks, but the uplift of the rift shoulders is not very important. The following ‘typical’ rift formation stage is associated with normal faults bordering the main tilted blocks, while subsidence of the rift floor and uplift of the shoulders are important. The advanced rifting stage, shown by important magmatic intrusions along the rift axis, corresponds to the initial formation of oceanic crust. These stages correspond to different present day aspects of various rift segments in Eastern Africa.

A proglacial origin for the upper Proterozoic Rockfish Conglomerate, central Virginia, U.S.A.

The upper Proterozoic Rockfish Conglomerate is a lenticular unit of stratified pebbly sandstone and conglomerate as much as 500 m thick at the base of the Lynchburg Group in central Virginia. It has been interpreted as a deep-water resedimented deposit, but glacial influence is suggested by the presence of dropstones and coarse-grained rhythmites. The coarsest deposits in the Rockfish Conglomerate are confined to the lower 30 m and consist of stratified pebbly sandstones containing outsized clasts and lenses of cobble conglomerate. They are interpreted as subaqueous glacial outwash deposited on a delta front. Coarse-grained, rhythmically-bedded sandstones containing thin, laterally persistent pebbly horizons make up the bulk of the unit and probably accumulated by a combination of mass flow and ice-rafting processes seaward of the delta front. Overlying this rhythmite facies is a thin horizon of dropstone-bearing turbidites. The Rockfish Conglomerate may reflect local progradation of highland glaciers across the margin of the Lynchburg basin during the initial stages of late Proterozoic rifting. Recognition of glaciogenic rocks in the Lynchburg Group provides further evidence for regional glaciation in the southern Appalachians during late Proterozoic time. Glaciation in the southern Appalachian Blue Ridge may correlate with the worldwide Vendian glacial event, compatible with geochronologic data that indicate rifting between 690-570 Ma in the southern Appalachians.

Tectonics and Dispersal of Miocene Rift-Fill Sediments, Southern New Mexico: ABSTRACT

Late Tertiary rifting in southern New Mexico took place in two stages: early to late Miocene and Pliocene-Pleistocene. Sedimentary rocks of the earlier stage are represented by the Hayner Ranch and Rincon Valley Formations, which were examined at San Diego Mountain, Dona Ana County, New Mexico. The Hayner Ranch Formation consists of 950 m of red fanglomerate derived from volcanic rocks. The Rincon Valley Formation consists of 325 m of limestone- and volcanic-cobble fanglomerate overlain by 25 m of red playa shale. Measurements of maximum clast size at 50 m intervals (n = 1,820) and cobble imbrications at 50 m intervals (n = 650) indicate that the initial stage of rifting took place in two distinct phases and involved two separate source areas. The lower 600 m of the Hayner Ranch Formation were derived from volcanic rocks north of San Diego Mountain. Following a period of tectonic quiescence, represented by a 150 m thick fining-upward sequence, uplift occurred to the south of the study area, depositing the upper 350 m of the Hayner Ranch Formation and the lower 300 m of the Rincon Valley Formation. Limestone cobbles recovered from the Rincon Valley Formation are identical to lithologies in the Permian Hueco formation exposed in the Robledo and Dona Ana Mountains, 15 km south and southeast of San Diego Mountain, respectively. End_of_Article - Last_Page 314------------